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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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426                                 ALASKAN GLACIER STUDIES
melted away, there will be a conspicuous ridge of rocky material of the sort interpreted elsewhere as crevasse deposits.
Where the ground moraine forms the southern shore of the lake there are several capes and bays, and large valleys and kettles, apparently corresponding to former lobes in the ice edge. Here cliffs rise 40 or 50 feet above lake level, and the slope continues steeply below water, as is shown by the fact that in one place 375 feet from the shore the lake is 150 feet deep, while in another place it is 45 feet deep 50 feet from shore, indicating that these are ice-contact slopes rather than wave-cut cliffs, though above lake level they have been somewhat modified by wave work.
Oidwash Plain. The southern edge of the terminal moraine is fringed by a series of small alluvial fans of coarse gravel leading out of little ravines in the moraine. A few hundred feet from the moraine they merge into a smooth, gently-sloping outwash plain which extends southward and westward. This plain is trenched by scores of sub-parallel, southwest-trending, abandoned stream courses which form the only irregularities in the outwash plain, with the exception of two groups of hills of older moraine.
The border between the moraine and the outwash plain is generally sharp, but in places the moraine is so low that only the nature of the soil indicates the boundary. This is true for a short distance southeastward from the railway bridge where the till and large sub-angular bowlders of the moraine, in contrast with the assorted rounded gravel and smaller stones of the outwash plain, are more useful than the topography in mapping the moraine.
Vegetation on Moraine and Outwaah. The vegetation in this southern area formerly occupied by the Miles Glacier bulb, covers the outwash plain, the terminal moraine, and part of the ground moraine. On the outwash gravel plain there is a thick growth of cottonwood and dense underbrush, with trees a foot or two in diameter, the oldest one we found, a cottonwood, having 70 annual rings. In the abandoned stream channels which extend across the outwash plain, there are no trees as old as those between the stream courses. The alluvial fans at the base of the terminal moraine are covered with moss and grass but have no mature forest like that on the outwash plain.
The terminal moraine is about as densely covered with vegetation as the outwash plain, but there are no cottonwoods. The woody plants are all alders and willows and they form such a complex network of close-set, inclined, interlacing trunks that travel through the thicket is very difficult. The plants are younger than those on the outwash plain, the oldest, whose age we determined, having 20 annual rings.
In the belt of ground moraine the vegetation is progressively younger along a line from the terminal moraine along the lake shore, toward the ice edge the ages of the bushes decreasing from 19 to 5 years. Acres of moraine, too rocky for the rapid growth of bushes, were still absolutely barren in 1910, but wooded slopes extend out to the lake shore, and an alder with 19 annual rings was found within a quarter mile of the glacier. There were no shrubs near the retreating edge of the detached ice block, but some small shrubs were growing in the ablation moraine upon its surface.
Evidence of Two Stages. The difference in ages of trees on the moraine and on the outwash plain calls for a special explanation. It is impossible that the ice stood at the terminal moraine up to something over 20 years ago, while the vegetation upon the outwash plain was growing freely, for in that case the trees upon the outwash plain should have been buried in gravels, uprooted, or killed by the water.